A normal ignition system for an internal combustion engine of a vehicle, etc. has, as shown in FIG. 10A, an ignition plug 10, a battery 31, an ignition switch 32, an ignition coil 33, an electronic control unit (ECU) 35, an igniter (transistor) 34, and the like
In this system, as shown in FIG. 10B, when the ECU 35 generates an ignition signal after the ignition switch 32 is turned on, the igniter 34 turns on so that a voltage (e.g., 12 V) of the battery 31 is applied to a primary winding 331 of the ignition coil 33 causing a primary current in the primary winding 331. When the ignition signal disappears and the igniter 34 turns off, the primary current is cut off causing a magnetic field change in the ignition coil 33. A secondary winding 332 of the ignition coil 33 generates a secondary voltage of −10 to −30 kV in response to the magnetic field change. This secondary voltage generates electric discharge in a discharge space (gap) 140 between a center electrode 110 and a ground electrode 131 in the ignition plug 100, so that a high temperature zone is locally formed in a limited area. At this moment, a current of about 35 mA, which is rectified by a rectifier (diode) 21, flows in the secondary winding 332, and energy of about 35 mJ is discharged. In normal spark ignition by the ignition plug 10, this high temperature zone becomes a source of ignition to ignite compressed air-fuel mixture supplied to a combustion chamber of the engine for mixture explosion.
A plasma ignition system for an internal combustion engine of a vehicle, etc. also has, as shown in FIG. 11A, an ignition plug 10, a battery 31, an ignition switch 32, an ignition coil 33, an electronic control unit (ECU) 35, and an igniter (transistor) 34, as the normal system shown in FIG. 11A. In addition, the plasma ignition system has a plasma generating power circuit 4, which includes a battery 41, a resistor 42, a plasma generating capacitor or capacitors 43 and a rectifier 22. The ignition plug 10 is a plasma type, which includes an insulator 120 surrounding a center electrode 110 and defining a discharge gap 140. The capacitor 43 is provided to store electric energy by being charged by the battery 41 through the resistor 42.
In this plasma ignition system, as shown in FIG. 11B, a secondary voltage of about −10 to −30 kV is generated in the similar manner as in the normal ignition system. In addition, at a moment when the secondary voltage reaches a discharge voltage proportional to a discharge distance D between the electrodes 110 and 131, the energy stored in the capacitor 43 is instantly discharged in the discharge space 140 so that high temperature plasma gas in the discharge space 140 is generated. At this moment, high energy of about 100 mJ is emitted.
In this plasma ignition system, a relatively large high temperature zone is formed by very high energy and becomes a flame kernel of high directivity, which ignites compressed air-fuel mixture in an engine. Thus, the plasma ignition system is expected to be applied to a stratified combustion in a direct-injection engine, in which lean air-fuel mixture is combusted by supplying rich air-fuel mixture only around the ignition plug.
Since the energy stored in the capacitor 43 is supplied to the ignition plug 10 instantaneously, a large current of about 120 A flows in the negative direction during a discharge period of about 8 μs as shown in FIG. 10B. This occurs at every predetermined rotation of the engine, and hence high frequency electromagnetic noise N is generated. This noise is likely to cause various electronic control systems mounted in a vehicle. This may lead to misfire in the engine.
To counter this electromagnetic noise, U.S. Pat. No. 4,308,488 (JP-U-55-156263) proposes to form an electric wire of a plasma generating circuit and provide a steering diode in this electric wire at a position close to an ignition plug. This proposal will not cause reduction in a voltage supplied to a primary winding of an ignition coil from a discharging power circuit.
Since the shield wire has low flexibility, wiring the shield wire becomes difficult. If the shield wire has an imperfectly shielded part, electromagnetic noise leaks. As a result, the other electric wire of a discharging power circuit and a plug cap need be shielded. This shield may not be easily provided in a crowded engine compartment. In some instances, this shield itself operates as an antenna and generates electromagnetic noise. Further, since the stray capacitance formed between the shield and the electric wire of the plasma generating power circuit changes irregularly in accordance with bending, this may result in a new source of noise.
Further, the ignition coil and the plasma ignition plug is likely to form a transmission circuit, which generates electromagnetic noise when the ignition plug starts to discharge in response to the secondary voltage of the ignition coil. The electric wire is likely to operate as an antenna and radiates the noise outward. Since a large current must flow in the electric wire, it is not possible to stop generation of electromagnetic noise, which is generated at a start of discharging, by a resistor in the electric wire.